With the rapid development of digitization of information and other information technologies, a style that is becoming widespread is to distribute various kinds of contents information, such as videos and sounds, at a high speed while maintaining a high grade for the user to utilize the received information at his end. An information utilization pattern that is often found at the user's end is to save the distributed data temporarily, for example, in a hard disk or the like and then to accumulate information in another recording medium by choosing and editing the information the user wishes to save for a long period. In order to meet an explosive increase of an information volume and a change of the information utilization style for the high-grade information as described above, there has been a need for a system capable of realizing ultra-high speed recording and reproduction and an ultra-large capacity.
As one of promising means to satisfy the requirements above, there has been proposed a hologram optical information recording/reproducing device (also referred to as a hologram memory and a holographic memory) utilizing photon mode optical information recording. The hologram recording/reproducing method is a method to record a signal light beam modulated according to data to be recorded and a reference beam in a hologram recording medium in the form of an interference fringe and to reproduce data recorded in the hologram recording medium by irradiating a readout beam (reproducing reference beam) that is equivalent to the reference beam to the hologram recording medium in which the data is recorded.
As the hologram memory optical information recording system described above, for example, there is a hologram memory optical system adopting shift multiplexing recording proposed by Psaltis et al. In the hologram memory optical system adopting shift multiplexing recording, the beam diameter of light from the laser light source is expanded by a beam expander and the light is divided to two beams by a half mirror. One of the divided beams passes through a spatial light modulator and forms a signal light beam as it is focused on a hologram recording medium by a Fourier transform lens. The other beam serves as the reference beam that irradiates the hologram recording medium at the same position as the signal light beam. A hologram medium, such as photopolymers, is encapsulated in a space between two glass substrates of the hologram recording medium, and an interference fringe of the signal light beam and the reference beam is recorded in the hologram medium.
The spatial light modulator is formed of optical switch arrays aligned two-dimensionally and the respective optical switches are switched ON and OFF independently in response to an input signal to be recorded. For example, in a case where a spatial light modulator formed of 1024 cells×1024 cells is used, 1 M bits of information can be displayed simultaneously. When the signal light beam passes through the spatial light modulator, 1 M bits of information displayed on the spatial light modulator is converted to two-dimensional light beam arrays and recorded on a hologram recording medium in the form of an interference fringe. When the recorded signal is reproduced, the reference beam alone is irradiated to the hologram recording medium and a diffracted beam (referred to also as reproduced beam) from the hologram is received at a two-dimensional image capturing portion, such as a CCD element.
The hologram memory optical information recording system is characterized in that the hologram medium is as thick as about 1 mm and information in the form of an interference fringe is recoded as a thick grating, that is, a so-called Bragg grating, which enables multiplexing recoding and hence a large-capacity optical recording system. This system realizes angle multiplexing by shifting the irradiation position of a spherical-wave reference beam instead of changing the incident angle of the reference beam. In other words, this system exploits the fact that when the recording position is shifted by slightly rotating a disk-shaped medium (a hologram recording medium), the incident angle of the reference beam sensed at the respective portions of the medium changes slightly.
Given that the thickness of the hologram medium is 1 mm, then the wavelength selectivity defined by the intensity of a reproduced signal is found to be the full width at half maximum of 0.014 degree. By multiplex-recording holograms at an interval of about 20 microns when the NA of the reference beam is 0.5 and the hologram size is 2 mmφ, the recording density achieved in this case is 600 G bits/inch2, which is 730 GB of a disk capacity in a 12-cm-diameter disk.
As other multiplexing recording methods, for example, it is possible to multiplex-record a plurality of holograms in one recording region of a hologram recording medium by using means for changing the incident angle of the reference beam or the phase distribution of the reference beam each time one piece of two-dimensional information is recorded in the form of a hologram. These multiplexing recording methods make it possible to record data at extremely high density and these methods have a feature that a recording capacity can be enhanced dramatically in comparison with conventional optical disks (compact disks abbreviated as CDs, digital versatile disks abbreviated as a DVDs, and so forth). In addition, these methods make it possible to record and reproduce two-dimensional information displayed on the spatial light modulator at a time. Extremely high-speed data access in comparison with CDs and DVDs can therefore be achieved.
For a hologram recording medium, various materials, including an organic material, such as photopolymers, and an inorganic material called photorefractive crystals, are proposed. Studies and developments have been carried out extensively from the viewpoints of the basic characteristics of materials, such as the recording sensitivity, the recording capacity, and the information retaining capability, as well as from the viewpoints of the manufacturing method and the cost.
Because the hologram recording/reproducing method is an information recording/reproducing method using a photon mode of light, the hologram recording medium is a so-called photosensitive material by nature of the recording method and it has sensitivity to light at a wavelength not exceeding the visible light range. Accordingly, when a non-recorded region of the recording medium is exposed to light before the recording, crucial characteristic deterioration, such as a reduction of a recordable capacity, can occur. In practice, however, in order to facilitate the handling, photon mode optical information recording media, such as a hologram recording medium, contain a reaction retardant. These media therefore have a non-linearity with respect to the photosensitive characteristic and remain unexposed under low-power light irradiation conditions.
Meanwhile, the multiplexing recording/reproducing method, which is one of the most noticeable features of the hologram recording/reproducing method, has been also developed vigorously. For example, as is disclosed in Patent Document 1, there is angle multiplexing by which multiplexing recording and reproduction are carried out at the same region by changing the incident angle of one or both of the reference beam and the signal light beam with respect to the hologram recording medium, and as is disclosed in Patent Document 2, there is peristrophic multiplexing by which multiplexing recording is carried out by rotating the incident directions of the reference beam and the signal light beam with respect to the normal line to the hologram recording medium. In angle multiplexing described above, the incident angle can be changed by mechanical means, such as a galvanometer mirror, and electric means, such as a deflector using an acousto-optic element or an electro-optic element.
In addition, as still another multiplexing recording method, Patent Document 3 discloses polytopic multiplexing characterized in that a plurality of adjacently multiplexed holograms that will be reproduced simultaneously are filtered using an aperture of the like so that a reproduced beam by substantially one hologram alone is retrieved. Further, as is disclosed in Patent Document 4, there has been proposed to combine angle multiplexing and peristrophic multiplexing by constituting luminous flux deflecting means and deflection control means from a wedge-shaped prism and rotational operation means for rotating the prism.
The proposals of the hologram recording/reproducing method including the proposals of the multiplexing methods as described above are chiefly to achieve an enhancement of the recording capacity by increasing the multiplicity in the hologram recording medium, or to remove factors that lowers the diffraction efficiency, such as erasing of holograms caused by multiplexing recording, and therefore provide the principle underlying the multiplexing recording in the hologram recording medium.
Incidentally, as with the optical disks, such as CDs and DVDs, it is necessary to check (or verify) whether the recorded data (two-dimensional information) is the desired data also in the information recording and reproduction by the hologram recording/reproducing method. To this end, Patent Document 5 proposes a direct read after write (hereinafter, abbreviated as DRAW) function. This function exploits the fact that a diffracted beam (that is, a reproduced image) from the currently recorded interference fringe can be obtained because once the recording of an interference pattern in the hologram recording medium is started, the recording reference beam serves also as the reproducing reference beam. Patent Document 5 describes that it is possible to reproduce or check the currently recorded information simultaneously with or immediately after the recording by retrieving the reproduced image using a CCD or the like.
However, in the case of using an organic material, such as photopolymers, which is a hologram recording material of a write once read many (abbreviated as WO or WORM) type generally used extensively, in the hologram recording/reproducing method for carrying out photon mode recording, for example, optical recording is carried out by utilizing a change of the refractive index during polymerization from monomer to polymer. In this instance, there is a time zone (called a dark reaction) in which molecules migrate over a certain time after irradiation of light until a refractive index distribution that should be obtained in the end is formed inside the hologram recording medium. Accordingly, the dark reaction has to be completed (a certain time has to elapse) in order to reproduce the information. This poses a problem that it is substantially difficult to achieve information recording at a high transfer rate by reproducing and verifying the recorded information simultaneously with the recording of information or immediately after the recording. This is a problem arising from the dark reaction, which is an essential physical phenomenon known as a behavior of the hologram recording material. It is therefore difficult to solve this problem by the proposal disclosed in Patent Document 5.
Also, in a case where the information is reproduced and checked after the recording by taking the time needed for the dark reaction into account, because it is necessary to irradiate the reproducing reference beam, it is difficult to reproduce and check the recorded information without deteriorating the recordable capacity of the hologram recording medium in a state where the multiplicity in an arbitrary region of the hologram recording medium has not reached a desired target value (the feasible maximum multiplicity). Hence, in order to avoid deterioration of the recording capacity, the recorded information has to be checked after the recording to the feasible maximum multiplicity ends in at least an arbitrary region in which multiplexing recording is carried out. For example, in a case where the sequence to check the recorded information by saving the recorded information temporarily in a buffer memory equipped inside the device is adopted, a volume of data to be saved increases with an increase of the recording capacity by increasing the multiplicity as a capability of the hologram recording medium. This poses a problem that a large buffer memory is required. Also, in a case where no buffer memory is provided, it is necessary to check the recorded information by reading out the recorded information again. This poses a problem that the time needed for the checking becomes longer.
In the hologram recording materials used for discussion by the inventors of the present application to date, there is a phenomenon that the sensitivity of the recording material changes with the progress of the multiplexing recording (an increase of the multiplicity). However, there is no technique to understand a change of the recording sensitivity while the recording is taking place. Hence, there is a problem that it is necessary to understand in advice the recording sensitivity characteristic of the hologram recording medium to be used for particular recording scheduling (for example, arrangements regarding the light exposure time and recording intervals when the N′th hologram is recorded during multiplexing recording) through conformation by actually consuming a part of the hologram recording medium to be used.
Also, it is necessary to ensure the characteristic of a hologram to be recorded by correcting the recording scheduling in response to a change of the recording sensitivity when the recording state changes due to a variance of the in-plane characteristic of the hologram recording medium and a characteristic change of the light source and the recording optical system. However, there is a problem that the incapability of understanding the recording sensitivity of the hologram recording material in the multiplexing recording region makes the correction difficult. Further, as with the foregoing, it is also necessary to confirm the recording sensitivity that varies from one material sample to another and deterioration with time of the recording sensitivity characteristic by carrying out test recording to understand the recording sensitivity.    Patent Document 1: JP-A-2003-43904    Patent Document 2: U.S. Pat. No. 5,483,365    Patent Document 3: JP-A-2004-272268    Patent Document 4: JP-A-2000-338846    Patent Document 5: Japanese Patent No. 3652338